Genetic archives

If you start from two pairs you produce progeny that are not sibs of each other. If you produce from a single trio all progeny are related to the male (they are all brothers and sisters). If you start out with two pair you not only decrease your initial inbreeding in the next generation, but you increase your potential genetic diversity by 25%.

It is common in the fancy to maintain a line with at least two separate breeding pens. If you mate the progeny from the different pens together you are assured that they are not sibs. Inbreeding is generally a bad thing, but it remains the best way to breed the traits that you want into your birds. The reason to mate relatively unrelated birds, but still related is that your selection progress can still be observable and you decrease the chance that inbreeding depression will run your line into extinction.

Inbreeding increases more rapidly for sib matings than for cousin matings. Eventually the two lines will produce cousins on a perpetual basis. This is an oversimplification but full sib matings increase the inbreeding by 25%, half sib matings by 12.5%, but first cousin matings increase inbreeding by 6.25%. The difference may not seem too great, but it is very substantial when you consider chance fixation rates. Fixation is just the chance that all birds have the same allele (are homozygous) inherited from some single ancestor. Infomaniac may have a simple explanation of why the fixation rates are so different, but the rate at which the fixation rates rise to equal the inbreeding rates is probably a couple orders of magnitude between an inbreeding rate of 6.25% and one of 25%. If the fixed allele is detrimental (bad for the bird) your line must now live with this disadvantage because you can't breed it out. Fixation means that there is no longer variation at that gene to select on. If too many of these detrimental alleles are fixed your line will die out. If you slow the rate of fixation you increase your chances of selecting against these detrimental alleles and maintaining you line.

CJR calls this braiding the lines. If you have three lines you can decrease inbreeding even more, but your genetic gains will be expected to be less for any trait that you are selecting for.

Line breeding, where you are crossing parent to offspring, is one of the fastest ways to inbreed, but also the fastest means to breed for the traits that you want to have in your birds. The disadvantage is that any bad genes that the superior parent has, become very common in your line and have a high probability of being fixed by chance. Line breeding relies on the chance fixation of the traits that you want in your birds, but it dies by the chance fixation of the recessive bad genes. This is why some breeders tell you not to line breed, it often results in good show birds for a few generations, but the line usually becomes reproductively weak and new blood has to be brought in that usually wipes out most of the gain by line breeding.

There is no doubt that line breeding is the most effective breeding technique to produce the birds that you want if you know what you are selecting for, so it may be best to maintain a braided line and branch off a few extra pens to experiment with line breeding.